1. Field of the Invention
The present invention relates to an alkaline storage battery with a hydrogen storage alloy for negative electrode and a process for producing the hydrogen storage alloy particles.
2. Description of the Related Art
Strenuous effort has been expended to develop an metal oxide-hydrogen alkaline storage battery using a negative electrode of a hydrogen storage alloy and a positive electrode of a metal oxide such as nickel oxide, since such battery can be made to have a higher capacity than that of the conventional nickel-cadmium storage battery.
This type of battery, however, suffers from a reduction in cell capacity upon the repetition of charging and discharging, because the surfaces of the negative electrode of the hydrogen storage alloy are oxidized with oxygen evolved at the positive electrode at the time of overcharging to dimimish the hydrogen storage ability of the alloy.
At the time of charging, the hydrogen storage alloy causes a hydrogen storage reaction and a hydrogen evolving reaction represented by the following equations 1 and 2, respectively:
Charging EQU M+H.sub.2 O+e.fwdarw.MH+OH.sup.- (M:hydrogen storage alloy)(1) EQU H.sub.2 O+e.sup.- .fwdarw.OH.sup.- +1/2H.sub.2 .uparw. (2)
Therefore, when the battery is charged, especially quickly charged at a larger current density, the internal pressure rises owing to the oxygen gas generated at the positive electrode and the hydrogen generated at the negative electrode.
This increased internal pressure of the cell causes the relief valve attached to the cell to operate releasing gases associated with a leak of the electrolyte out of the cell resulting in an reduction in the amount of the electrolyte. Thus, the charge and discharge cycle characteristics of the cell decline.
In this circumstance, the oxygen evolved at the positive electrode reacts with the hydrogen stored in the hydrogen storage alloy to be reduced forming water, though the hydrogen evolved at the negative electrode itself is apt to be accumulated in the cell.
An attempt has been proposed to overcome the problems as above by a technique of coating the surfaces of the hydrogen storage alloy with nickel (Ni) or nickel alloys as disclosed in Japanese Patent KOKAI (Laid-open) No. 61-163569. This technique prevents the hydrogen storage alloy from contacting with the oxygen to inhibit the oxidation of the alloy as well as to impart an electroconductive to the alloy, thereby achieving a higher efficiency in charging and discharging.
Japanese Patent KOKAI (Laid-open) No. 2-204965 proposed a technique where powdery hydrogen storage alloy and a powdery metal such as tin (Sn), copper (Cu) and lead (Pb) were mixed to improve the overall electric conductivity of the negative electrode, thereby enhancing the charge and discharge cycle life property.
Still another technique has been proposed to allow the hydrogen absorption reaction as expressed by the formula (1) as previously described to be effectively proceed by providing a material acting as catalyst to the hydrogen absorption reaction on the surfaces of the hydrogen storage alloy.
In order to effect an efficient absorption of the hydrogen evolved at the negative electrode into the hydrogen storage alloy itself at the time of charging, another technique has been proposed to enhance the solid-gas phase reaction between the alloy and the hydrogen over the hydrophobic area of the alloy by coating a hydrophobic material on the surfaces of the alloy.
A powder of such material was mixed with the hydrogen storage alloy particles, on the surfaces of which the material was to be applied, in a pulverizer such as a ball mill. When this mixing was conducted, base particles of the hydrogen storage alloy were coated on their surfaces with finer particles of the aforementioned powdery material by adhesion thereof.
Alternatively, a technique of coating the surfaces of the hydrogen storage alloy by plating has been partly employed.
The alloy particles produced by the techniques as described above, however, suffered from weak strength in bonding the base particles of the hydrogen storage alloy and the finer particles of the powdery material, especially physical bonding strength therebetween.
When the cells with such hydrogen storage alloy used as negative electrode are repeatedly charged and discharged, the hydrogen storage alloy particles undergoes repeatedly expansion and contraction with the charging and the discharging. This expansion and contraction of the alloy particles dimimishes further the physical bonding strength between the base particles and the finer material particles, which may cause separation of the latter from the surfaces of the former to achieve only insufficient effect with the coating of the finer material particles.
For this reason, even with the powdery material as described above applied on the powdery hydrogen storage alloy, the cells have only limited charge and discharge cycle characteristics.